The present invention relates to the field of thermoelectric devices, and more particularly to thermoelectric devices for the direct generation of electric power from a heat flux (Seebeck effect) and pumping of heat using applied electric power (Peltier effect).
Current commercialized thermoelectric elements are typically bulk materials with thermoelectric figure of merit (ZT) values around one. New thin film thermoelectric materials based on metal-semiconductor (M-S) multilayers are currently being developed in order to improve ZT to levels that will make thermoelectric power generation more practical. Maximum power densities for such M-S multilayers are achieved when thermoelectric leg thicknesses are on the order of 50-200 μm. However, due to constraints associated with standard thin film deposition techniques (e.g. sputtering), these M-S multilayers have practical thickness limits of 10-20 μm. In addition, optimal M-S multilayer film quality is often achieved for thickness values less than 10 μm.
Mayer and Ram's work (Mayer P M and Ram R J, “Optimization of Heat Sink-Limited Thermoelectric Generators”; Nanoscale and Microscale Thermophysical Engineering, Volume 10, Issue 2, pages 143-155, July 2006) highlights some of the challenges in using thin-film thermoelectric materials. They point out that the leg length must be on the order of about 100 microns, depending on the heat transfer coefficients, in order to reach the maximum power point. Any thinner and the performance will be limited by the inability to maintain the temperature difference, and/or by the parasitic electrical interface resistance.
Metal-semiconductor multilayers are currently being explored as new thermoelectric materials with high thermoelectric figures of merit (ZT). These metal-semiconductor multilayers are typically deposited by reactive DC sputtering, a thin film deposition technique suitable for industrial scale-up.
Due to the finite heat transfer coefficients of available heat sinks, thin-film based thermoelectric power generators demonstrate maximum power density at specific leg lengths. For example, forced water convection and heat-sinking technology have optimal leg lengths of 400 μm and 20 μm, respectively. The 20-400 μm range is impractical for sputtering high quality M-S multilayers, which means a disconnect exists between the thermoelectric thickness requirements for maximum power density and thickness limits for high-quality sputtered thin films. This disconnect can be overcome using laminated M-S multilayers disclosed herein.